Methodologies and test configurations for testing thermal interface materials

ABSTRACT

Methodologies and test configurations are provided for testing thermal interface materials and, in particular, methodologies and test configurations are provided for testing thermal interface materials used for testing integrated circuits. A test methodology includes applying a thermal interface material on a device under test. The test methodology further includes monitoring the device under test with a plurality of temperature sensors. The test methodology further includes determining whether any of the plurality of temperature sensors increases above a steady state.

FIELD OF THE INVENTION

The invention relates to methodologies and test configurations fortesting thermal interface materials and, in particular, to methodologiesand test configurations for testing thermal interface materials used fortesting integrated circuits.

BACKGROUND

The high cost associated with manufacturing integrated circuits dictatesthat defective devices be diagnosed as early as possible in themanufacturing process. For this reason, it is highly advantageous totest integrated circuit devices at the wafer level before furtherprocessing and packaging. For testing, the wafers are clamped to a waferchuck and brought into contact with highly sophisticated probes. Onceprobe contact is made, the circuits are tested for both functionalityand power integrity. However, circuits can be tested at power levels inexcess of 400 watts, which consequently generates a tremendous amount ofheat build-up in the chips which must be removed by the massive waferchuck.

After wafer test, the wafers are diced into individual devices alsoknown as chips. The good devices are mounted onto substrates to createmodules. The modules then go through additional testing that may includeburn in and re-test. The test equipment typically includes a highperformance heat sink, test socket and tester electronics. The module isinstalled in the socket, the heat sink is brought into contact with thechip, the chip is tested, the heat sink is removed and the module isremoved from the socket and sorted based on functionality. Some or allof these steps are commonly automated.

In some applications, a Liquid Thermal Interface (LTI) is placed betweenthe chip and heat sink in order to improve thermal contact. There aremany different types of LTIs that can be used to improve the thermalcontact such as, for example, a mixture of water and other additives(i.e., Propylene Glycol (PG)). Water has excellent thermal performancebut at high test temperatures may evaporate before the end of test.Also, water and other substances such as, for example, PG, can corrodethe C4 connects. The corrosion or evaporation often happens only onsmall areas of the device and only on a small number of the totaldevices tested in a high volume manufacturing environment.

Alternate interface materials can have other disadvantages. Helium isclean and non-corrosive but thermal performance is insufficient. PAO(Poly Alpha Olefin) oil and various types of thermal grease have thermalperformance almost as good as water but require cleaning with a solventafter test. Thermal pads have insufficient thermal performance and oftenleave a residue. Fluorinated fluids may be non-corrosive and clean buthave poor thermal performance due to their low thermal conductivity.Liquid metals and soft metals can have very good thermal performance butcan oxidize (degrade) over time and repeated reuse, they can damage theheat sinks and can have unreliable thermal contact to the chip. Greases,phase change materials and adhesives can make it difficult to separatethe heat sink from the chip after test and leave a residue.

If faults stemming from the improper use of LTI are found afterproduction has started it may be necessary to recall product and developa new LTI resulting in significant production delays. Thus there is aneed for improved methods of evaluating LTI fluids to ensure they willprovide reliable service. Accordingly, there exists a need in the art toovercome the deficiencies and limitations described hereinabove.

SUMMARY

In a first aspect of the invention, a test methodology comprisesapplying a thermal interface material between a device and heat sink.Power is applied to the test device. The test methodology furthercomprises monitoring the device with a plurality of temperature sensors.The test methodology further comprises determining whether any of theplurality of temperature sensors increase above a steady statetemperature.

In another aspect of the invention, a test methodology comprisesmeasuring conductivity between two interconnects by applying a voltagebetween the two interconnects. The test methodology further comprisesapplying a thermal interface material in contact with at least twointerconnects. The test methodology further comprises determiningwhether there is an increase in current when the thermal interfacematerial is in contact with the two interconnects.

In yet another aspect of the invention, a test methodology comprisesplacing a quantity of thermal interface material on a device under test.The test methodology further comprises placing a glass slide at apredetermined load on the thermal interface material. The testmethodology further comprises determining whether the thermal interfacematerial adequately covers the device under test without running over anedge; and at least one of if the thermal interface material does notadequately cover the device and/or runs over the edge, then clean theglass and the device and reapply a different quantity of the thermalinterface material, and repeat the placing the glass and determiningsteps; and if the thermal interface material adequately covers thedevice then accept the thermal interface material.

In still yet another aspect of the invention, a test configurationcomprises a substrate or card for attaching to a device. The testconfiguration further comprises a heat sink structured to dissipate heataway from the device. The test configuration further comprises a powersupply which powers the device. The test configuration further comprisesa plurality of temperature sensors at locations throughout the deviceunder test and which take temperature readings at the locations. Thetest configuration further comprises a temperature measurement devicefor determining an evaporation of a thermal interface material betweenthe device under test and the heat sink by sensing a temperature riseabove a steady state temperature of one or more of the temperaturesensors.

In yet another aspect of the invention, a test configuration comprises apower supply which applies a voltage between interconnects. The testconfiguration further comprises a current measurement device configuredto measure current passing through the interconnects and a thermalinterface material which contacts the interconnects. The currentmeasurement device provides an indication of a difference in currentbetween a voltage applied to the interconnects without the thermalinterface material and when the thermal interface material is in contactwith the interconnects.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The present invention is described in the detailed description whichfollows, in reference to the noted plurality of drawings by way ofnon-limiting examples of exemplary embodiments of the present invention.

FIG. 1 shows a flow diagram of method for testing thermal interfacematerials in accordance with aspects of the invention;

FIG. 2 a shows a configuration using temperature sensors to test athermal interface material in accordance with aspects of the invention;

FIG. 2 b shows a test socket with a module having temperature sensors,with a heat sink in non-thermal contact with the device in accordancewith aspects of the invention;

FIG. 2 c shows a test socket, where the heat sink is in thermal contactwith the module having temperature sensors in accordance with aspects ofthe invention;

FIG. 2 d shows a plurality of temperature sensors in accordance withaspects of the invention;

FIG. 3 shows a temperature performance graph of a known thermalinterface material obtained using the methods of the present invention;

FIG. 4 shows a temperature performance graph of a thermal interfacematerial under a temperature test for 1 hour, obtained using the methodsof the present invention;

FIG. 5 is a graph showing data from running a test methodology numeroustimes at different chip temperatures obtained using the methods of thepresent invention;

FIGS. 6 a and 6 b show temperature performance data of different thermalinterface materials at several different temperature sensor positionsalong the device obtained using the methods of the present invention;

FIG. 6 c shows temperature performance data of different thermalinterface materials as a function of the material thermal conductivityobtained using the methods of the present invention;

FIG. 6 d shows the result of a simple model predicting temperaturegradients on a product module that may have variable LTI coverage andpower;

FIG. 6 e shows thermal data measured on production modules;

FIGS. 7 a and 7 b show alternative test configurations to measureconductivity (and observe thermal interface material) in accordance withan additional aspect of the invention;

FIG. 8 shows a graph representative of a corrosion test (current vs.time) for various thermal interface materials in accordance with aspectof the invention;

FIGS. 9 a-9 c show representative devices that used thermal interfacematerials during testing, in accordance with aspects of the invention;

FIG. 9 d shows chip surfaces at different phases of testing;

FIGS. 10 a and 10 b show a test configuration to observe thermalinterface material on a surface of the device in accordance with anadditional aspect of the invention; and

FIG. 11 shows a flow diagram of a method for testing thermal interfacematerials in accordance with aspects of the invention.

DETAILED DESCRIPTION

The invention relates to methodologies and test configurations fortesting thermal interface materials and, in particular, to methodologiesand test configurations for testing thermal interface materials used fortesting integrated circuits. The methodologies herein provide improvedtechniques for determining evaporation of the thermal interfacematerials (LTI), as well as corrosion caused by using different LTIs.

FIG. 1 shows a method of testing a device to ensure that a thermalinterface material (LTI) will provide adequate heat transfer propertiesto the device under test. For example, using the test configurationshown in FIGS. 2 a-2 d, the method of the present invention includes, atstep 100, providing a controlled weight and volume of an LTI to thedevice or heat sink. In embodiments, the weight and volume depends onthe tooling attached for the test. The weight or volume for a givendevice/surface size is calculated or estimated on the chip size, chipand heat sink flatness, chip test temperature and test time. Forexample, too little weight/volume will not give complete coverage forthe entire test; whereas, too much weight/volume will be a waste ofmaterial, which may spill out of the active area.

At step 105, a heat sink is applied to the device with a predeterminedload, and temperature. The load is provided to adequately make contactbetween the heat sink and device and thermal interface material. Inembodiments, the heat sink load and geometry will be similar to theproposed production manufacturing application. In embodiments,temperature, time and force may be, for example, 115° C. for 60 minutesat 10 lb. It is understood, though, that this is merely one non-limitingexample, as other examples are provided herein. At step 110, a knownpower is applied to the device to heat the device to a temperature,e.g., 115° C. At step 115, the method monitors a plurality oftemperature sensors that are integrated into or coupled to the testdevice (or production device, if available) to determine thetemperatures at various locations. The method will assume that thetemperature sensors will reach a steady state temperature as discussed,for example, with reference to FIG. 3.

In embodiments, the temperature sensors can be small resistors which arecalibrated in an oven by measuring electrical resistance vs temperature.A straight line is fit to the calibration data. During testing of theLTI, temperature is calculated from the measured resistance. Other typesof temperature sensors such as diodes can also be used with the presentinvention.

At step 120, the process determines if the temperature sensors havereached a steady state. If the temperature sensors have not reached asteady state, at step 122, the process determines if any of thetemperature sensors exceed a safe limit, e.g., 140° C. depending on thematerials of construction of the test device and the specification forproduct test. If the temperature is exceeding a safe limit, the test isended and restarted with new conditions of heat sink temperature, LTIvolume and/or power. If the temperature does not exceed a safe limit,the process returns to step 115.

If the steady state is achieved at step 120, the process then proceedsto step 125. At step 125, the method notes the time at which a firsttemperature sensor increases above a steady state as the first evidenceof evaporation of the LTI from the device interface. The test ends ifthe temperature increases above the steady state; whereas, it willcontinue until the temperature increases above the steady state.

In embodiments, during manufacturing, the method looks for a change inaverage vs. change in maximum temperature. In embodiments, temperaturereadings may be made (monitored) before, during and after power isapplied to the chip in order to provide comparison and base line data.However, the present invention should not be limited to any specifictime periods for temperatures monitored before, during and after poweris applied to the chip; although, in embodiments, the test may be runlonger than the anticipated product test time, with the time varyingfrom product to product.

Table 1, below, shows examples of weights, loads, wattage, test time andtest temperature.

TABLE 1 Chip LTI Test Test Chip Size Power Weight Load Time Temp Examplemm × mm Watt mg lbf min ° C. 1 14.7 × 14.7 320 2.1 12.5 60 115 2 20.9 ×21.7 75-350 4.0 3.5 15 85

FIGS. 2 a-2 d show a configuration for implementing a test methodologyaccording to an aspect of the invention. The test methodology using theconfiguration shown in FIGS. 2 a-2 d will be used to determine whether athermal interface material (LTI) can withstand test conditions, e.g.,from about 5 to 50 minutes at about 55° C. to about 115° C., withoutevaporating.

More specifically, FIG. 2 a shows a test chip module with temperaturesensors and heaters in accordance with aspects of the invention. Thestructure 10 includes a primary electronic component (device) 15 thatincludes interconnects (e.g., C4s) 20 for bonding to any known type ofsubstrate 25 and/or card. The device 15 can be a single component (e.g.,chip) or may be a combination of components (e.g., lidded module). Inembodiments, the device 15 may or may not be attached on the substrate25 and/or card that may or may not have secondary electronic components45 with or without interconnects 30. The test device 15, in embodiments,is to be used to test and evaluate LTI materials. In embodiments thetest chip module has a similar size to the production modules that willeventually be tested for performance, yield and/or reliability with theLTI.

The module may also contain an underfill material surrounding the C4interconnects (not shown) and may be covered with a lid (not shown). Innon-limiting embodiments, the device 15 (or chip) can have dimensions21.3 mm×26.7 mm with C4 interconnects powered up to at least about 340W. In another illustrative, non-limiting embodiment, the device or chipcan have dimensions 23.6 mm×22.0 mm with C4 interconnects powered up toat least about 400 W.

FIG. 2 b shows a test socket with the test chip module of FIG. 2 a, witha heat sink in non-thermal contact with the device in accordance withaspects of the invention. FIG. 2 c shows a test socket, where the heatsink is in thermal contact with the test chip module having temperaturesensors in accordance with aspects of the invention. In both FIGS. 2 band 2 c, temperature sensors 60 are integrated into or coupled to thedevice 15 to measure the temperature of the test device, and determineproperties of the LTI.

Referring to both FIGS. 2 b and 2 c, the socket may include a substrateclamping feature 100 and a hinged 35 b cover 35 a that incorporates aspring 35 c loaded heat sink 35. As shown in FIG. 2 b, the module isplaced into the socket and LTI 40 placed thereon. The cover 35 can thenbe closed as shown in FIG. 2 c. In particular, the socket includes aprimary electronic component (device or chip) 15 that includesinterconnects (e.g., C4s) 20 for bonding to any known type of substrate25. The device 15 can be attached to a ceramic substrate 25 havingcapacitors as secondary electronic components also attached to thesubstrate 25 with lead tin or lead free interconnects. The device 15 isin thermal contact with a heat sink 35 by an LTI 40, in order to takeaway (e.g., dissipate) heat from the powered device. The LTI 40 can bein direct contact with both the device 15 and the heat sink 35.

In embodiments, the heat sink 35 may be a water cooled heat-sink with anickel plated surface to take away heat from the powered device 15. Theheat sink 35 may be placed in contact with the device 15 by a heat sinkactuator 35 a, hinge mounted 35 b to a test board 35 d. The heat sinkactuator 35 a may include a spring mechanism 35 c to provide a downwardpressure on the heat sink 35. The heat sink 35 may include flexiblesupply tubing or bellows to supply cooling fluid while allowinggimbaling and flexibility (not shown). The heat sink 35 may also includeheating elements and temperature sensors to actively control chiptemperature. It is understood that FIG. 2 c is representative only,there are many other types of sockets, heat sinks and heat sinksactuators well known in the art. It is common that some or all parts ofthe process are automated. The spring loaded heat sink is forced intodirect contact with the device at some small area of contact. Thereremains a small gap across the rest of the area which is caused by thenon flat surfaces and will be affected by the heat sink force. This gapis filled by the LTI.

FIGS. 2 b and 2 c further show a plurality of temperature sensors 60integrated into or coupled to the thermal test device 15 (or actualproduction device, if available). In embodiments, the temperaturesensors 60 can be small resistors or diodes (or combination of resistorsand diodes). In other embodiments, it is possible to use the actualproduction device as a thermal test device because modern microprocessordevices often already have multiple temperature sensors. In a test chip,resistors are designed across the chip to result in a uniform or atleast known pattern of power dissipation. It should be understood,though, that the pattern of power dissipation across the product deviceis not as well known as in the test device. When one area of a productdevice starts to overheat that area will generally start drawing morecurrent from the rest of the chip resulting in an increase in powerdensity which, in turn, increases temperature. This positive feedbackcan result in localized thermal runaway and damage to the device.Additionally, the production device may not be available in the timeframe of the LTI testing.

FIG. 2 c shows a power supply 65 and temperature measurement device 70.In embodiments, the power supply 65 provides power levels in excess of400 watts, which consequently generates a tremendous amount of heatbuild-up in the device 15. The temperature measurement device 70 isconnected to the temperature sensors 60, for determining the temperatureacross the device 15. In embodiments, the temperature measurement device70 monitors the temperature sensors 60, to determine a steady state andan increase in temperature of any of the plurality of temperaturesensors 60. More specifically, the temperature measurement device 70 isstructured and arranged to determine an evaporation of a thermalinterface material 40 between the device 15 and heat sink 35 by sensinga temperature rise above a steady state temperature of one or more ofthe temperature sensors 60. The temperature measurement device 70 canalert the user of these different states by use of, for example, analarm, display or other user interface.

The temperature sensors 60 can be provided in a predetermined pattern onthe device as shown, for example, in FIG. 2 d. More specifically, inFIG. 2 d, the temperature sensors S1-S12 are embedded in the device (orcoupled to the device) in order to obtain various temperature readings.In embodiments, the temperature sensors can be located at many differentlocations, all at known locations for measuring the temperature of thedevice under test. For example, one contemplated configuration would beto place a temperature sensor in each corner and one in the center for aminimum of five (5) temperature sensors. Also, although 12 temperaturesensors are shown in FIG. 2 d, it is well understood that more than 12temperature sensors are advantageous because small areas of a chip maystart to dry out before any temperature effect is seen on distantsensors. In another contemplated configuration, 27 temperature sensorswere used to measure a temperature at various locations on the chip.

In embodiments, the test device is installed in the test socket, LTIfluid of known volume is applied and it is contacted by the heat sink ina manner for the production of devices. The test device is connected tothe power supply 65 which supplies a known voltage and current. Thepower in watts is the product of the measured voltage and current. Thepower supply 65 can include a current measurement device configured tomeasure current passing through the interconnects and a thermalinterface material which contacts the interconnects. In embodiments, thecurrent measurement device provides an indication of a difference incurrent between a voltage applied to the interconnects without thethermal interface material and when the thermal interface material is incontact with the interconnects.

The temperatures are monitored before, during and after power is appliedto the chip using the temperature measurement device 70, connected tothe temperature sensors 60. As previously noted, the present inventionshould not be limited to any specific time periods for temperaturesmonitored before, during and after power is applied to the chip.

If the device is known from its design to have a uniform power acrossits area, it is expected to see a fairly uniform temperature gradientacross the device. The temperatures should quickly settle in at a steadystate value for as long as constant power is applied. Steady state is atemperature increase over time that is inconsequential to the expectedproduct test results. As a practical example, a temperature increase ofless then 1° C. in 60 seconds may be considered steady state. The steadystate temperature increase is often divided by the power to obtain athermal characterization parameter that is typically referred to as athermal resistance; units are C/W or may be further normalized by thedevice surface area as C sq cm/W. This calculation may be performed foreach temperature sensor as well as the average of all temperaturesensors. In this methodology, if one corner (or other location) becomeshotter than the others, it can be deduced that there is insufficient LTIat that corner (or other location). Other possibilities that would beinvestigated are a non-flat heat sink or non uniform heat sink force.After the steady state thermal resistance is determined the test iscontinued to see if and at what time there will be some evidence ofevaporation.

Typical results using the methodologies and configuration describedherein are shown in FIGS. 3-6 c and 6 e. FIG. 3 shows results of a knownthermal interface material mixture of water and propylene glycol testedbetween a 19 mm×20 mm chip and a liquid cooled heat sink with a coolingfluid temperature of 60° C. The heat sink force was 44 lb. At time=0,the power is off and the device is at the cooling fluid temperature, andwhen the power is turned on the chip heats up. In this test, there are11 temperature sensors across the chip. As shown, the temperatures reacha steady state temperature (as indicated by the horizontal lines) withinabout 30 seconds of applying power. This steady state remains for about300 seconds when one corner sensors start increasing in temperature;most of the other temperature sensors remain at this steady statetemperature for the duration of the test. Other sensors start toincrease within about 400 seconds of when power is applied, indicatingthat the fluid in the area of those temperature sensors has dissipated.Accordingly, using the test configuration of the present invention, ithas been found that an increase in temperature is due to fluidevaporation from between the chip and heat sink. This would beconsidered an acceptable test only up to about 300 seconds. This testmay have some variability each time it is run. In embodiments, thepresent invention contemplates applying a factor of safety to suchmeasured data when using the measured data to develop a highly reliableprocess.

FIG. 4 shows a temperature performance graph of an improved thermalinterface material (e.g., Pentadecane) under a temperature test for 1hour, in accordance with aspects of the invention. More specifically,FIG. 4 shows a temperature test for Pentadecane (C₁₅H₃₂) as measured atseveral locations along a device under test. These locations arerepresented by “ctr” (center) and “b” through “h”, which are positionsof temperature sensors along a device under test. This graph shows thatPentadecane was able to withstand temperatures of about 115° C. forabout an hour, without evaporating. This is shown by the steady state(e.g., constant) lines in the graphs, compared to the profile of FIG. 3.The test was ended after 1 hour because that is about 3 times longerthan the expected production test time. The 3× factor of safety willensure a highly reliable manufacturing process.

The data shown in FIG. 5 is from running the test methodology numeroustimes at different chip temperatures. The time to begin evaporation iswhen any temperature sensor first starts to increase above the steadystate temperature. This plot predicts the maximum “safe” test time for agiven type and volume of LTI. The shape of the curve will also bedependent on other factors such as chip size and heat sink flatness.

FIG. 6 a is an example of results done for a large number of TIM and LTIcandidates measured with one test chip, heat sink and force. It shouldbe understood by those of skill in the art that results will bedifferent with other test chips, heat sinks or forces. Plotted areresults from the hottest, coolest and average sensors. FIG. 6 b showsthermal resistance data obtained from testing 5 different product chipseach with three (3) different TIM materials. At one point in the testprogram the power and temperatures are measured. As expected thetemperature sensors in core processor areas are hotter than sensors inother areas of the chip. As shown in the graph, helium providesnon-acceptable thermal properties. It is also known that helium needsconstant replenishment or a very well sealed test chamber, as it willdissipate during test. Water (e.g., PG/water), on the other hand, isshown to have good thermal properties. Pentedecane provides good thermalproperties, and will not evaporate during test.

FIG. 6 c shows the results of several tests of different LTI materialsusing the same test chip, heat sink and force. Normalized averagethermal resistance (C sq cm/W) is plotted vs the interface resistivity(which is inverse thermal conductivity) in units of C sq cm /W. The datais linear. A simple thermal model predicts that the slope of the line isthe average LTI gap thickness of 0.00011cm or 1.1 microns. The yintercept is a prediction of overall heat sink performance with aperfect interface material (infinite thermal conductivity). This type ofchart can be used to predict performance of other LTI materials if theirthermal conductivity is known or to estimate their thermal conductivityif resistance is known. Helium was found to have worse thermalperformance than predicted from this chart because the mean free pathapproaches the gap size, thus effective thermal conductivity of heliumin these small gaps is about 2× lower than bulk published values. Thismethod of analyzing the data also provides a way to estimate the effectof heat sink and device flatness on thermal performance.

FIG. 6 d shows the result of a calculation that assumes each quadrant ofan idealized chip is independently powered and cooled. Resistance isbased on the total power and temperature rise of the hottest, coolestand average sensor. The first five cases assume uniform power in eachquadrant. The first case has uniform LTI, as expected the temperature isuniform at 0.09 C/W. In case 2, LTI is removed from one quadrant and itstemperature increased to 0.21 C/W. As LTI is removed from the otherquadrants, the temperature of those quadrants increases to 0.21 C/W thusincreasing the average temperature. The second of the five cases assumeuniform LTI coverage and a total chip power of 100 watts. The first ofthese cases has 25 watts on each core. The next case has 40 watts in onecore and 20 watts in each of the other cores resulting in an increasedtemperature of the 40 watt core. However, the average temperatureremains the same. The same trend is seen as the power becomes more andmore non uniform. It is known that production chips can occasionallyhave manufacturing defects where one core draws more power than theothers.

FIG. 6 e shows data measured on production modules at one point inmodule test. The device has 9 temperature sensors. The maximum, averageand minimum temperature increase is plotted versus total module powerfor each device (shown as a box with error bars). There is a largevariation in chip power for these modules but the average sensor data ingeneral follows a linear trend. An action limit based on average isshown (12). The action limit is 1.5 C at zero power due to the accuracyof the temperature sensors. Modules with an average above the actionlimit (11) were tested without LTI due to a tool malfunction. Modules 10have good LTI but are defective chips that allow one core to drawexcessive power. This data analysis method suggested by the model ofFIG. 6 d can be used to determine if a production device suffers fromnon uniform LTI (resulting in an increase in average temperature) or nonuniform power (resulting in acceptable average but increase in maximumtemperature). Alternately, if the power of each core is known, thermalresistance can be based on the temperature rise of each core divided bythe core power resulting in an improved measure of the presence of LTI.

FIG. 7 a shows a test configuration in accordance with an additionalaspect of the invention. The method using the configuration of FIG. 7 acan measure conductivity of the LTI under conditions close to what willbe used when testing product with LTI in the unlikely scenario wheresome of the LTI runs over the chip and gets into the area of theinterconnects. In other words, conductivity is measured between adjacentinterconnects. Ideally the interconnects are the same alloy, size andspacing as the interconnects used in the product, as an example 0.005″diameter solder balls 97% lead, 3% tin spaced 0.009″ apart. Theadvantage of the method of FIG. 7 a is to be able to observe corrosionor electromigration dendritic growth between the interconnects in thepresence of LTI.

In the test configuration of FIG. 7 a, a substrate 25 with solder balls(interconnects) 20 is covered with a glass slide 80. (In embodiments,the device may also be mounted to the substrate 25, however, this willnot allow viewing of the interconnects or the movement of LTI among theinterconnects when LTI is introduced,) The substrate 25 is placed withina test socket 35 b, which may be mounted to a PCB test card 90. Voltagepotential is provided between two or more interconnects 20 by the powersupply 65. The current is monitored, and an LTI fluid candidate 40 isintroduced into the area at the edge of the glass slide 80 where it willtypically wick under the glass slide 80 and wet the interconnects 20. Itshould be noted that the LTI fluid candidate 40 may be provided on thesubstrate 25, prior to applying the glass slide 80 or providing thecurrent as shown in FIG. 7 a. In embodiments, the method can verify alow initial current flow, apply a drop of LTI sufficient to cover(between or in contact) the at least two interconnects 20, and recordcurrent vs. time. An increase in the current indicates an unacceptableLTI candidate. Also, observation of corrosion through the glass slide 80indicates an unacceptable candidate, as discussed in more detail below.The results can be sent to a computing device 100 for storage and futureretrieval and/or analysis.

For unacceptable candidates, the current may continue to increase overtime due, possibly, to one or more phenomenon. For example, the LTI maybecome more conductive in the presence of air or due to contamination onthe interconnects 20 or device. Alternatively, or in addition, theconductive metal dendrites may grow between adjacent interconnects 20,effectively creating a short circuit. It has also been observedconductivity may increase and decrease over time, which may be due todislocation of the dendrites, dry out of the LTI and evolution of gas,e.g., Hydrogen will be released from water in the presence of sufficientvoltage.

FIG. 7 b shows an alternate embodiment where the device 15 is placed ona stage 59, and two adjacent interconnects 20 are contacted byelectrical probes 57. The probes 57 are connected to a power supply 65,and current is measured. It should be understood that this embodimentmay not use a glass slide, even though direct observation is not quiteas clear without the glass slide 80. By way of an example, one probe canbe connected to a ground interconnect, while a second probe can beconnected to an I/O interconnect. In embodiments, the probes can beconnected to other solder connections, such as, for example, dummy C4s(solder connections). Once the probes are applied, the power source canapply about 1V-3V. In this example, about 3 μl of solvent (LTI) can beapplied to the device. The device can be monitored with a parameteranalyzer such as, for example, an HP 4145B Parameter Analyzer(manufactured by Hewlett Packard™). In embodiments, the HP 4145BParameter Analyzer can take about 1000 data points/scans, where a singlescan is about 55 seconds. In embodiments, a “medium” scan is about 73seconds (each point average of 16 measurements) and “long” scan is about323 seconds (each point average of 256 measurements). The results of thescans can be sent to a computing device 100 for storage and futureretrieval and/or analysis.

The computing device 100, as should be understood by those of skill inthe art can include an operating system, a processor, ROM, RAM andpermanent storage such as, for example, an internal hard drive or anexternal storage such as, for example, a CD ROM, magnetic storage,optical storage of the like. More specifically, the computing device 100includes a processor, memory, an I/O interface, and a bus. The memorycan include local memory employed during actual execution of programcode, bulk storage, and cache memories. In addition, the computingdevice 100 includes random access memory (RAM), a read-only memory(ROM), and an operating system (O/S). The computing device 100 is incommunication with the external I/O device/resource and storage system.The processor executes computer program code (e.g., program control),which can be stored in the memory and/or storage system. While executingthe computer program code, the processor can read and/or write datato/from memory, storage system, and/or I/O interface. The bus provides acommunications link between each of the components in the computingdevice. The functionality provided by the computing device 100 can beimplemented by a computing article of manufacture that includes anycombination of general and/or specific purpose hardware and/or computerprogram code. In each embodiment, the program code and hardware can becreated using standard programming and engineering techniques,respectively.

FIG. 8 shows a graph representative of a corrosion test (current vs.time) for various thermal interface materials in accordance with aspectof the invention. More specifically, FIG. 8 shows a graph of data for acorrosion test (current vs. time) for various solvents, includingPentadecane (C₁₅H₃₂) and Hexadecane (C₁₆H₃₄) in accordance with aspectsof the invention. This test shows Pentadecane and Hexadecane arenon-corrosive materials to the C4, as well as exhibit good electricallyinsulating properties.

Other materials that exhibit good thermal properties without causingcorrosion include, for example, Nerol, Limonene, Pinane and Squalene.These materials may, however, evaporate during the test and/or leaveresidue on the device under test. For example, as discussed below, Nerolmay leave a residue on the device under test, which would requirecleaning with solvent. It has been discovered from this test procedurethat all materials that had oxygen showed varying amounts of currentflow and corrosion. Surprisingly, materials that did not contain oxygendid not allow current flow and had no sign of C4 corrosion.

FIG. 9 a shows corrosion residue left after testing a device with waterbased LTI after some LTI came into contact with the chip C4s duringelectrical test. FIGS. 9 b and 9 c show devices using Pentadecane 15 aand Hexadecane 15 b, respectively, that do not exhibit any residue on oraround the chip C4s after testing.

FIG. 9 d shows chip surfaces at different phases of testing withdifferent thermal interface materials, including Pentadecane inaccordance with aspects of the invention. The testing with the interfacematerial disposed between (in contact with) the chip and heat sink isfor 30 min at 85° C. More specifically, FIG. 9 d shows data for fourthermal interface materials (PG (Propylene Glycol)/water (morespecifically, 30% PG+70% De-ionized water), Nerol, Pentadecane andFarnesol). After 30 minutes at 85° C., the heat sink is removed and allchips except that with PG/Water are 100% covered by the thermalinterface materials. The excess thermal interface material is removed bysuction in a de-applicator and then the parts were baked uncovered in anoven for 20 minutes at 70° C. The exposed fluid evaporates much quickerthan when it was covered by the heat sink.

Pentadecane shows many advantages over the remaining materials, one ofwhich is no staining after the uncovered baking in an oven for 20minutes at 70° C. This is in comparison to PG/Water, Nerol and Farnesol,each of which present issues at the same temperature. For example, asshown by the data of FIG. 8 d, Nerol leaves a 3% stain; whereas,Farnesol leaves a 70% residue at 70° C.

FIGS. 10 a and 10 b show a test configuration to observe LTI on thesurface of the device in accordance with an additional aspect of theinvention. In the test configuration of FIGS. 10 a and 10 b, a device(chip) 15 is mounted to a substrate 25 and placed within a test socket35 b, which may be mounted to a PCB test card 90. An LTI 40 is dispensedonto the device 15, and covered with a glass slide (plate) 80 todisperse the fluid across the surface of the device. The glass slide 80is ideally optical quality and of same dimensions and flatness as theheat sink. Alternately, a common optical flat can be used. Alternatelythe test may be done without the test socket and PCB card.

In embodiments, the LTI 40 is placed on the device 15 and the glassslide 80 is contacted to the device 15 in the same manner and force asthe heat sink, for example. In one example, the force of the glass slideon the device may depend on the design of the test equipment. Forexample, a force can range from about 4 to 45 psi, and more specificallyabout 10 psi. Looking down through the glass slide 80 will allow anobserver to determine (view) the extent of the LTI coverage on thedevice 15. With some types of LTI, it is common to see the fluidcoverage change over time as the fluid migrates across the interfacebetween the device and glass slide 80.

It should be understood that FIGS. 10 a and 10 b are not to scale, andthe actual thickness of the LTI 40 is variable across the chip and isdetermined by the fluid properties, the flatness of the glass slide 80and device 15, and the force with which they are pushed together. Actualmaximum LTI thickness is typically on the order of 10 microns for a baredie and about 50 microns for lidded devices. FIG. 10 a shows anunacceptable coverage of the LTI 40. For example, in this case the LTI40 has not covered the entire chip on the right side and is in danger ofrunning down the left side. FIG. 10 b shows a top view of the testconfiguration with an unacceptable coverage. In this view, it isobserved that the LTI does not cover the corner of the chip.

FIG. 11 shows a method of determining whether a quantity of LTI issufficient to cover the device under test. The method of FIG. 11 can usethe test configuration of FIGS. 10 a and 10 b, for example. At step1100, a drop of known size of LTI is placed on the device. At step 1105,a glass slide is placed over the LTI and compressed with the same forceas the test heat sink, for example. At step 1110 and 1115, it isobserved (determined) whether the LTI fully covers the device and doesnot overflow. If satisfied, at step 1120, the LTI is accepted, and atstep 1130, the process establishes an acceptable process window. If theprocess window determined in step 1130 is small relative to the accuracyof the dispense tool, the LTI is rejected. The accuracy of the dispensetool may be on the order of 0.2 mg. If step 1115 is not satisfied, theglass and device are cleaned and the process is repeated with analternate volume at step 1125. If dispenser tolerance is such thatcomplete coverage cannot be obtained without overflow, the LTI isrejected at step 1135.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements, if any, in the claims below areintended to include any structure, material, or act for performing thefunction in combination with other claimed elements as specificallyclaimed. The description of the present invention has been presented forpurposes of illustration and description, but is not intended to beexhaustive or limited to the invention in the form disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the invention.The embodiments were chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

What is claimed:
 1. A test configuration, comprising: a substrate orcard for attaching to a device under test; a heat sink structured todissipate heat away from the device under test; a power supply whichpowers the device under test; a plurality of temperature sensors atlocations throughout the device under test and which take temperaturereadings at the locations; and temperature measurement device todetermine an evaporation of a thermal interface material between thedevice under test and the heat sink by sensing a temperature rise abovea steady state temperature of one or more of the temperature sensors. 2.The test configuration of claim 1, wherein the plurality of temperaturesensors are resistors or diodes or a combination of resistors anddiodes.
 3. The test configuration of claim 1, wherein the device undertest is a thermal test chip.
 4. The test configuration of claim 1,wherein the plurality of temperature sensors are positioned at eachcorner of the device under test and at a central location of the deviceunder test.
 5. The test configuration of claim 1, further comprising athermal interface material on the device under test.
 6. The testconfiguration of claim 1, wherein the temperature measurement devicerecognizes a time at which a first temperature sensor increases abovethe steady state as first evidence of the evaporation of the thermalinterface material from an interface between the device under test andthe heatsink.
 7. The test configuration of claim 1, wherein thetemperature measurement device makes temperature readings before, duringand after power is applied to the device under test in order to providecomparison and base line data.
 8. The test configuration of claim 1,wherein the heat sink is in non-thermal contact with the device undertest.
 9. The test configuration of claim 1, further comprising a testsocket for holding the device under test, wherein the heat sink is inthermal contact with the device under test.
 10. The test configurationof claim 9, wherein the test socket includes a substrate clamp, and ahinged cover that incorporates a spring loaded heat sink.
 11. The testconfiguration of claim 10, wherein the device under test is placed inthe test socket and the thermal interface material is provided in directcontact with the device under test and the heat sink.
 12. The testconfiguration of claim 11, wherein the heat sink is a water cooled heatsink with a nickel plated surface to take away heat from the deviceunder test.
 13. The test configuration of claim 12, wherein the heatsink includes a spring mechanism to provide a downward pressure on theheat sink.
 14. The test configuration of claim 1, wherein the powersupply provides power levels in excess of 400 watts, which generates aheat build-up in the device under test.
 15. The test configuration ofclaim 1, wherein the temperature measurement device alerts a user ofdifferent temperature states by use of one of an alarm and display.